An energy storage-type capacitor is a capacitor having a mechanism which may store energy while serving as a conventional capacitor, and an energy storage device which may serve as a bridge between a battery and a capacitor. In terms of energy density and power density, an energy storage-type capacitor having intermediate characteristics of an electrolytic condenser and a secondary battery is a system which has a shorter charging time and a longer service life, and may achieve higher output than a secondary battery, and has a 10-times higher energy density than a conventional electrolytic condenser. In general, in the capacitor, a positive electrode and a negative electrode, which are manufactured by applying each of the electrode materials on each of the electrode current collectors, are coupled to each other, having a separator therebetween, and the capacitor configured of the positive electrode/the separator/the negative electrode is received in various gaskets and then an electrolytic solution is injected thereinto, thereby manufacturing a final capacitor. The energy storage-type capacitor is an energy storage device that converts chemical reaction into electrical energy using electrostatic orientation (electrochemical double layer) of ions at the interface between the electrode and the electrolyte and stores the electrical energy. The capacitance (C) value in the capacitor is proportional to a contact area, and is inversely proportional to a distance between positive charges and negative charges, that is, the thickness of a dielectric layer. In the energy storage-type capacitor, the area of the energy storage-type capacitor is dramatically increased by using a nano-scale porous carbon electrode material, and the capacitance value may be increased to have ultrahigh capacity by decreasing the thickness of the dielectric layer to become an ionic layer of 10 Å.
The supercapacitor is classified into an electrochemical double-layer capacitor which stores charges in an electrochemical double layer of an interface between an electrode and an electrolyte, and a redox capacitor, also referred to as a pseudo capacitor, which is accompanied by changes in an oxidation number (valance) of transition metal ions on the surface of a transition metal oxide and stores charges or electrons, according to the operating principle.
Even though the electrochemical double-layer capacitor has a theoretically wide specific surface area using activated carbon, the area, which may be calculated and used as the actual capacitance value, is only 20 to 30% of the total specific surface area. This difference is related to ion size in the electrolyte to be adhered to the inside of the activated carbon and adsorption degree specifically, porous activated carbon may be classified into three types, macroporous activated carbon (20 Å<), mesoporous activated carbon (20 Å<pore size<100 Å), and macroporous activated carbon (>100 Å) according to the pore size, and among them, when the pore size is a micropore, the pore size may not be a size suitable for ions in the electrolyte to enter the pores. Accordingly, a large number of micropores in the activated carbon result in decreasing the dramatically increased specific surface area that is an advantage of using activated carbon. Accordingly, a method which may increase the power density of an energy storage-type capacitor is to maintain a porous structure suitable for a predetermined size of the electrolyte ion. However, this method incurs high costs and loss of time due to heat treatment several times and additional processes.
Meanwhile, when a single-type transition metal oxide is used for a redox capacitor, the resulting redox capacitor greatly deteriorates in terms of prices and efficiency. For example, RuO2 has proved to be currently best in terms of energy storage characteristics, but the prices are so high that RuO2 has a disadvantage in that RuO2 is not suitable for mass production, and a disadvantage in that the charging and discharging curve is non-linear in terms of efficiency. Accordingly, a material obtained in which a carbon-based material and a transition metal oxide are combined is used. For example, since carbon nano tube is advantageous in high electric conductivity and a wide specific surface area as a 1D-structure, but has a defect in that carbon nano tube shows a low unit volumetric capacity due to large voids, and a low theoretical capacity (372 mAh/g), which is the same as that of graphite, a result obtained by attaching a material such as SnO2, Sn, and SnSb, which exhibits high capacity to carbon nano tube has been reported [Y. Wang et al., Adv. Mat. 18 (2006) 645; G. An et al., Nanotech. 18 (2007) 435707; R. Li et al., J. Phys. Chem. C 111 (2007) 9130; M. S. Park et al., Chem. Mater. 19 (2007) 2406].
However, a metal oxide adsorbed on a carbon-based material has a size of about 10 to 100 nm, and thus, has a disadvantage in that the entire specific surface area is limited to 10 to 100 m2/g. Considering that a carbon-based material has a general specific surface area of 500 to 2,500 m2/g, it can be known that a substantial active area (contact area) is limited by the specific surface area of a small metal oxide.